The preparation of hydrocarbons from synthesis gas is well known in the art and is usually referred to as Fischer-Tropsch synthesis, the Fischer-Tropsch process, or Fischer-Tropsch reaction(s). Catalysts for use in such synthesis usually contain a catalytically active Group VIII metal supported on a carrier. The support may be alumina, silica, titania, zirconia, magnesia and their mixtures. The Group VIII metals, iron, cobalt, nickel, and ruthenium have been often used as the catalytically active metals. Cobalt and iron have been found to be most suitable for catalyzing a process in which synthesis gas is converted to primarily hydrocarbons having five or more carbon atoms. Additionally, the catalysts often contain one or more promoters. The catalyst promoter may be a basic oxide such as ThO2, La2O3, MgO, ZrO2, and TiO2, although promoters may also comprise noble metals such as Pt, Pd, Ru, Rh, Os, and Ir; coinage metals such as Cu, Ag, and Au; and other transition metals such as Fe, Mn, Ni, and Re. Fischer-Tropsch catalysts and processes are described in U.S. Pat. Nos. 6,476,085; 6,586,481; and 4,568,663. The active metal may be incorporated into the base by impregnation, drying and calcination as disclosed in U.S. Patent Application 2003/0162849 A1 to van Berge et al.
Alumina is one of the most desirable supports in catalysis. Due to its high surface area and good mechanical properties, particularly gamma form of alumina has been used widely in industry for many catalytic applications. However, in an acidic or alcohol containing reaction medium such as Fischer-Tropsch synthesis conditions to produce wax, or other reactions proceeding in aqueous medium such as alcohol, ether, and ester syntheses, an alumina support exhibits a stability problem. Alumina may dissolve or leach slowly in the reactor due to attacks of acid and alcohol byproducts in the reaction medium. Dissolution of alumina support in acid medium is expected to be detrimental in catalyst stability. The dissolution problem may cause poor maintenance of catalyst integrity and possible fines generation. Possible fines generation will hurt the subsequent filtration and post processing operations. High metal or metal compound content in a Fischer-Tropsch product is undesirable because such contaminants could have adverse effects for the Fischer-Tropsch process, such as causing reactor plugging or significantly reducing catalyst life. As a result, it is important that the product of the Fischer-Tropsch process be free of metal and other contaminants that could adversely affect its subsequent processing.
Thus it is highly desirable to have a catalyst support with the favorable characteristics of an alumina support with much improved acid resistance.
U.S. Pat. No. 5,733,839 to van Berge et al. shows that silica coating on gamma alumina particles improves dissolution tendency of alumina and the silica-coated alumina support is useful as a Fischer-Tropsch synthesis catalyst. U.S. Patent Application 2003/0162849 A1 to van Berge et al. shows that silica-coated alumina prepared via in situ coating in a slurry of aluminum hydroxylate prior to spray drying is also useful as a support for a Fischer-Tropsch synthesis catalyst. Daniell et al. studied the latter material in depth using FT-IR and XPS and showed that little mixing of the two oxide phases take place on the surface so the surface can be depicted as comprising two separate oxide phases (Applied Catalysis A: 196, 247-260, 2000).